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Simply, Catholics are not permitted to marry outside of the Church is because to do so is a mortal sin, but the marriage itself is still valid by virtue of the contract - provided both spouses were free to marry.

Someone feel free to correct me, but because of the Marriage Contract, Catholics who are free to marry and marry before a justice of the peace or outside of the Church does not in and of itself automatically render the marriage null. Sinful but not null.

This is not correct.  Such marriage would be invalid.  This is the same issue you have with attempting to reduce heresy / schism to mortal sin.

Canon Law explicitly states that these marriages are invalid ... not merely illicit (i.e. sinful).

I dunno, you could be right, but I think Matrimony is a different case from Ordinations / Absolutions.

Ott says this:
This ^^ is what I was getting at - initially, the Church regards all marriages as valid, and if they are not valid She needs to declare them invalid.

I think there are probably some marriage minded trads out there who, having no luck finding another trad to marry, might start to think there is an open field of invalidly married potential spouses out there they can tap into. Or converts to tradition who married when they were prot or NO or whatever and think they see a way out of a bad marriage. But the Church always initially says the marriages are valid until She declares them invalid.

Your citation rejects the separation between the marriage and the Sacrament.

Regardless, however, the Decree on Armenians simply referred to the fact that the couple actually contract the marriage, rather than the priest, at least in the Latin Rite (it's different in the Eastern).  This decree on the Armenians was referring to the schismatic Armenian Church, so the question was whether marriage contracted without the witness of a priest was valid for them (i.e. for these non-Catholics) or whether, upon being readmitted to the Church they would be considered valid.  This was to distinguish those couples among these schismatic non-Catholics who got officially married before a priest.   So the Church declares that these marriages among non-Catholics did not require the witness of a priest, i.e. not only did the Church accept as married those among them who had their marriages witnessed by a priest, but also those who did not, since the Church here declared here that 1) the couple contract the Sacrament of Matrimony and 2) the requirement for a priest to officiate or witness it does not apply to non-Catholics.